The present invention relates to reciprocating piston pumps and, more particularly, to high pressure, low flow rate pumps such as those used in high pressure liquid chromatography.
Reciprocating pumps used for precision liquid metering, such as HPLC, are usually comprised of a piston made from a brittle material such as sapphire or ceramic, and a seal made from a fluoropolymer compound. The piston is mounted on a piston rod that is driven from the distal end by a cam follower (push rod). The piston is usually made with a large length to diameter ratio, and is easily broken if care is not taken to provide for proper alignment in the design. Also, pistons are often broken during assembly or when the seals are changed unless great care is taken by the user during this process. Furthermore, any misalignment of the piston with the pump seal will cause rapid seal wear during operation.
In the prior art, pump manufacturers have relied on allowing the piston rod to float radially to prevent piston and seal misalignment. In this case, the seal is usually pressed into a recess in the pump head, and the seal is assembled to the piston as the head is mounted to the pump frame. There are many disadvantages to this method:
1. The seal is often damaged when it is pressed into the recess of the pump head. The diameter of the seal is always larger than the pump head recess into which it must be pressed, and unless the user is very careful, the seal will be damaged when it is pressed into place.
2. The piston is easily broken when the head is mounted to the piston. Once the seal has been successfully pressed into the pump head, the seal must then be pressed onto the piston in the same operation as the head is mounted on the pump chassis. This is a frequent cause of piston breakage because the head is rather heavy, and the force required to press the seal onto the piston can be quite high, so unless the user is extremely careful in aligning all components, the sapphire or ceramic piston will break.
3. The methods that are used to allow the piston rod to float are inherently unstable. The driving force for the piston rod is from the distal end. This end is usually either a rounded or flat surface that is supposed to slide on the push rod to permit a radial floating engagement. The proximal end of the piston depends on the seal, or a bushing behind the seal, for alignment.
The radial force required to align the piston in this case can be very large because of the moment arm that exists: (piston+piston rod length) X (axial load from fluid pressure) X (coefficient of friction) between piston end and push rod, and the seal or seal backup bushing is required to provide this force. The piston assemblies are usually required to be quite long to accommodate the stroke length and return spring, and the metal bearing at the piston rod end is often compromised by corrosion due to leakage during use with corrosive solvents, and by brinnelling over time. The result is large side loads on the seal and seal backup bearing, and short seal life.
Unfortunately, the general tendency toward rapid wear of the seal and seal backup bearing necessitates frequent component replacement, and each component replacement procedure increases the risk of breaking the brittle piston itself.
The present invention generally comprises a piston pump that is particularly adapted to provide high pressure output at low flow rates. A salient aspect of the invention is the provision of a floating seal arrangement that enables alignment of the seal with the piston, thereby reducing seal wear and improving the ease and success of assembly of the seal components in the pump.
The pump generally includes a pump chassis or body having one or more bores formed therein that are dimensioned to receive a piston rod. The piston rod is supported in the bore by precision bearings that virtually eliminate radial movement of the piston rod while permitting reciprocal axial translation. A piston extends axially from a proximal end of the piston rod, and is dimensioned to be received in a respective cylinder formed in a pump head.
The piston is sealed in the cylinder by a seal assembly that has both static and dynamic aspects. The static seal assembly includes an annular ring having a central aperture dimensioned to receive the piston in a leakproof sliding engagement, the ring being formed of a soft polymer material. The annular seal ring is encapsulated in an outer ring formed of a stiff plastic or polymer material, defining a self-contained seal cartridge. The outer ring is dimensioned to be received in the pump head in a cylindrical recess that permits positional freedom with respect to the axis of the cylinder.
The pump is assembled in a manner that assures alignment of the piston and seal cartridge while minimizing the risk of breaking the brittle piston. In the assembly process, the seal cartridge is first assembled to the respective piston, thus assuring that the seal is aligned with the piston. Thereafter, the head is assembled to the pump body, the pistons being inserted in their respective cylinders and the seal cartridges being received with clearance in their respective recesses. Subsequently, the head is tightly secured to the pump body, causing the seal cartridges to be immobilized in their recesses in the aligned relationships that have been previously established with respect to the pistons. The large radial clearance between the seal cartridge and the head recess eliminates side loading on the piston by the seal or head during installation, thus preventing piston breakage. In addition, it is not necessary to press the seal into the head, so that damage to the seal during installation is eliminated.